A brain function measuring device that uses a near-infrared spectroscopy (NIRS) may be used as medical and laboratory instruments or used for verification of educational effect or a rehabilitation effect, health management in homes, and market research such as product monitoring. Further, the near-infrared spectroscopy may also be used for a tissue oxygen saturation measurement or muscle oxygen metabolism measurement by using the same method. Further, the near-infrared spectroscopy may also be used not only for the measurement of sugar content of a fruit but also for a general absorption spectroscopic device.
The brain function measuring device using the near-infrared spectroscopy according to a related art includes an optical topographic method that noninvasively forms an image of a local hemodynamic change near a surface layer of a human brain. The optical topographic method irradiates light having a wavelength which belongs to a visible range to an infrared range onto a subject, detects the light which passes into the subject using a light detector at a point separated by several centimeters, and measures an amount of change in a hemoglobin concentration “(or change in multiplication of a hemoglobin concentration and an optical path”) to form a two dimensional image (for example, see Patent Literature 1 and Non-Patent Literature 1). The optical topographic method is less restrictive for the subject than a brain function measuring technology such as a nuclear magnetic resonance imaging (MRI) or a positron emission tomography (PET). In a clinical site, a verbal function or a visual function is measured.
In a photo detecting signal or a biological signal (hereinafter, referred to as an NIRS signal) obtained by a noninvasive optical brain function imaging using the NIRS including an optical topographic method, there is a report saying that since the light is irradiated from a scalp, a scalp blood flow change in the scalp may affect the photo detecting signal or the biological signal. A method that extracts and removes such a component has been studied in consideration of the influence of the scalp blood flow. Most of them obtain a signal component from portions having different depths by a method of a distance between a plurality of irradiators-detectors (optical transmitter-optical receiver) (hereinafter, referred to as SD (source detector) distance) and remove a scalp blood flow signal which may be considered to influence measurement data of shallow layers using the signal component. Hereinafter, a measurement method at a plurality of SD distances is referred to as a multiple SD distance method.
For example, there is a method that determines an absorption coefficient in a scalp and a brain (gray matter) by a simultaneous equation using an optical path length in the scalp and the brain (gray matter) in a short SD distance and a long SD distance (for example, see Non-Patent Literature 2). In the method, a head structure is assumed as a two layered structure and it is required to assume a partial mean optical path length of each of layers. However, it is difficult to estimate an optical path length of a subject.
Further, a subtraction method that uses an adaptive filtering is suggested. In the method, a value obtained by multiplying an appropriate coefficient by measurement data in the short SD distance (hereinafter, referred to as short SD distance data) is subtracted from measurement data in a long SD distance (hereinafter, referred to as long SD distance data) to remove a scalp blood flow signal (for example, see Non-Patent Literature 3). In addition, as a subtraction method that uses linear regression, a method that obtains a brain activity signal by subtracting a fitting signal that is obtained by linearly regressing the short SD distance data to the long SD distance data from the long SD distance data is suggested (for example, see Non-Patent Literature 4).
Following methods are disclosed as a technology related thereto.
In Patent Literature 2, in order to provide a photometric device that is capable of removing unnecessary information by scalp blood flow using an optical transceiver including a plurality of optical transmitting probes and a plurality of optical receiving probes, a method that disposes a pair of a plurality of irradiators and detectors at the same center point, performs measurement, and removes unnecessary information by an arithmetic processing is disclosed. Further, in Patent Literature 3, with a device configuration that uses two detectors for one light source, a method that appropriately distinguish information obtained from two detectors to obtain a result that mainly characterizes a state in a brain tissue without being influenced by an overlapped adjacent tissue is disclosed. In addition, in Patent Literatures 4, 5, and 6, a method that calculates change in absorbance and performs operation such as subtraction with the long SD distance data and the short SD distance data is disclosed. However, these methods have the following problems.
First, in the operation such as subtraction between measurement data in each of the SD distances, it is difficult to determine various coefficients. In such an operation, since the various coefficients may influence the result, it is required to set an appropriate value. Further, when the short SD distance data is obtained, since the SD distance is often set to be 10 mm or less and a signal component which depends on the change in the absorption only on the scalp but not the brain blood flow is obtained, an amplitude ratio of brain•scalp components is unknown. Therefore, it is difficult to determine an appropriate coefficient by the operation. In order to appropriately correct long SD distance measurement data including scalp contribution and brain contribution, there is a need to know a contribution ratio and an optical path length ratio of each of the scalp and the brain.
Further, when the short SD distance data is fitted to the long SD distance data, if the scalp blood flow signal and the brain blood flow signal are not independent from each other, that is, if the scalp blood flow signal is correlated with the brain blood flow signal, the brain blood flow signal may be undesirably removed from the long SD distance data.
As a method that does not use a multiple SD-distance method, a method that extracts the brain activity using a signal separating method is studied. For example, there is a study saying that a spatial homogeneity (broad spectrum) of independence components extracted using an independent component analysis (ICA) is indexed and if the homogeneity is high, simultaneously measured LDF signals show significantly high correlation (see Non-Patent Literature 5). In the study, without using information such as task time, the scalp blood flow is tried to be discriminated only using the independence and the spatial distribution of the NIRS signals. As related patents, in Patent Literature 7, a method that divides a signal into a plurality of independent components by the independent component analysis and removes unnecessary components using the broad spectrum thereof is disclosed. Further, in Patent Literature 8, a method that divides a signal into a plurality of independent components by the independent component analysis and removes the unnecessary components using a reference signal other than a brain function measurement signal is disclosed. The method is an analysis method based on an assumption that the scalp blood flow has a broad spectrum. If the assumption is not satisfied, the method cannot be applied. Therefore, in order to discriminate a signal from the brain and a signal from the scalp, robust and general analysis method and device configuration are required.